The present invention relates to an apparatus for and method of introducing gas into a molten metal. It is often desired to introduce gases into molten metals, for example in the degassing and cleaning of molten metal such as aluminum. Usually the gases are introduced through open ended tubes extending into the molten metal with their open ends located close to the bottom of a containing vessel such as a crucible or a gassing bay of a reverberatory furnace. It is known that gases are more effective for their intended purpose when introduced into the molten metal in the form of fine bubbles through some form of porous refractory brick or porous refractory member. When porous bricks are used, it is not practical to immerse these repeatedly into liquid metal since failure soon occurs due to thermal shock.
Better results have been obtained by building the porous components into the wall or floor of the containing vessel so that the components are well supported laterally against cracking. However, it is only a matter of time before the coarse components block up and fail to pass the gas at a satisfactory rate even when the gas is supplied under considerable pressure.
Good results have been obtained with porous refractory diffuser tubes made of porous graphite or low porosity graphite cemented to porous carbon and inserted horizontally through a wall of the containing vessel. However, these horizontally disposed tubes cannot be replaced without taking the container out of service, which is particularly disadvantageous if the container forms a part of a continuous process.
Diffusers of predominately carbonaceous material have other disadvantages. When used for introducing some gases into certain metals, for example, nitrogen into molten aluminum, they lead to fouling of the metal. It is difficult to make a gastight joint between the diffusers and the gas supply lines and the joints often fail through oxidation of the material. Attempts have been made to construct diffusers of shapes that would give more satisfactory introduction of the gas into liquid metal by making the diffusers of two or more parts, but the joining of the parts has presented difficulties, both initially and during use.
The method and apparatus of the present invention overcomes the above mentioned prior art deficiencies by supplying the gas through a supply conduit connected at one end to a source of pressurized gas and connected at the other end to a rotatable hollow sleeve immersed in the molten metal. An array of appertures extend through the sleeve for discharging the gas into the molten metal in the form of numerous gas bubble jets and the appertures are arranged to discharge the gas bubble jets in mutually cooperative jetting directions to rotationally propell the sleeve about the supply conduit. The rotary movement of the sleeve causes the gas bubble jets to whirl about in the molten metal, thereby agitating the molten metal to affect uniform dispersion of the gas within the molten metal.